Electrical color masking in a photo electrophoretic imaging process

ABSTRACT

The densities of pigments comprising an image are altered in electrophoretic imaging systems by supressing or enhancing pigment migration in an electric field by employing in selected areas electrostatic charge to increase or decrease the field effecting migration.

United States Patent 1191 Gundlach 1 1 ELECTRICAL COLOR MASKING IN APHOTO ELECTROPHORETIC A IMAGING PROCESS [75] Inventor: Robert W.Gundlach, Victor, NY.

[73] Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Feb. 27, 1970 [21] Appl. No.: 15,103

[52] U.S. C1. ..96/l.2, 96/1 .3, 96/1.4, 96/1 R, 355/3, 355/4 [51 Int.Cl. ..G03g 13/22 [58] Field of Search ..96/1,1.2,1.3,1.4;204/l81,204/299, 300

[56] References Cited UNITED STATES PATENTS 3,010,842 11/1961 Ricker..117/37 LE 14 1 Feb. 6, 1973 3,477,934 1 1/1969 Carreira et a1...204/181 2,986,466 5/1961 Kaprelian ..96/1 .2 3,535,221 10/ 1 970Tulagin ..204/181 3,565,614 2/1971 Carreira et al. ..96/l.4

Primary Examiner-Charles E. Van Horn At t0rney.lames .1. Ralabate, DavidC. Petre and Michael H. Shanahan [57] ABSTRACT The densities of pigmentscomprising an image are altered in electrophoretic imaging systems bysupressing or enhancing pigment migration in an electric field byemploying in selected areas electrostatic charge to increase or decreasethe field effecting migration.

7 Claims, 2 Drawing Figures PATENTED EB 6|975 SHEET NF 2 3,715,209

INVENTOR. ROBERT W. GUNDLACH DENSITY 400 s5o I --6 o I 760' WAVELENGTH,5;

ELECTRICAL COLOR MASKING IN A PHOTO ELECTROIIIORE'IIC IMAGING PROCESSBACKGROUND OF THE INVENTION This invention relates tophotoelectrophoretic imaging systems and in particular to methods andapparatus for correcting color in images produced by such systems. I

fluid. The charged'pigments or particles are attracted to either aninjecting or blocking electrode between the electrodes is transparent,preferably the injecting electrode, and the pigments adjacent theretoare exposed to electromagnetic radiation. Those pigments that absorb theradiation experience an apparent change in charge polarity and migrateunder the influence of the field toward the blocking electrode leaving apositive ink image on the injecting electrode and depositing a, negativeink image on the blocking electrode. The injecting electrode is so namedbecause it is designed to optimize the charge exchange with the pigmentswhile the blocking electrode is so named because it is designed tominimize the charge exchange with the pigments.

I Color images are normally made with the photoelectrophoretic processby exposing to the light emitted from an original flooded with whitelight an ink composed of yellow, magenta and cyan photosensitivepigments which ideally are exclusively absorbant of blue green and redlight respectively. The three pigments are positioned generally sidebyside and when viewed from a distance appear to the human eye as solidareas having colors depending upon the percentage of the variouspigments'in a unit area. Color correction-is necessary because presentlyknown pigments have nonideal responses to their respective portions ofthe visible light spectrum. Yellow pigments are generally closest to theideal absorbing substantially blue light only. Magenta and cyanpigments, unfortunately, deviate considerably from the ideal model.Magenta pigments primarily absorb green light but also absorb bluelight. Cyan pigments primarily absorb red light but also absorb blue andgreen light. Consequently, color correcting normally involves reducingthe density of yellow pigment in those areas also containing magenta andcyan pigments because too much blue light is otherwise absorbed in thoseareas. The quantity of yellow pigment removed is generally proportionalto the excess quantity of blue light being absorbed, i.e. proportionalto the density of the magenta and cyan in the same area. Similarly, thedensity of the magenta pigments is reduced in those areas alsocontaining cyan pigment becausev too much green light is otherwise beingabsorbed in thoseareas. Here, the quantity of magenta pigment removed isgenerally proportioned to the density of the cyan pigment.

. which the electric field is established. Normally, one of Photographiccolor masks are commonly used to effect the desired reduction in pigmentdensities, i.e. the color correction. The intensities or brightnesslevels of the blue, green and red light emitted from an original areinversely proportional to the density of the yellow, magenta and cyancolors in the original, respectively. Black and white photographicnegatives recording the intensities of the blue, green and red lightemissions, however, transmit light having intensities directlyproportional to the densities of the yellow, magenta and cyan areas inthe original. The three negatives are termed color separation negativesand the light transmitted by them is used to determine the location anddensity of the yellow, magenta and cyan pigments in a final colorreproduction. Black and white photographic positives made from the greenand red separation mega-- tives are the most commonly used color masks.The green and red separation positives are superimposed over the blueseparation negative and reduce the intensity of the transmitted light,i.e. the density of yellow pigment, in the yellow areas of the originalalso containing magenta and cyan. Similarly, the red separation positiveis superimposed over the green separation negative and reduces theintensity of the transmitted light, i.e. the density of the magentapigment, in the magenta areas of the original also containing cyan.Because of the amount of materials and the number of steps involved,these color correction techniques have proven expensive and timeconsuming.

Accordingly, it is an object of this invention to overcome thelimitations of known color correction techniques.

Specifically, it is an object of the present invention to simplify colorcorrection for electrophoretic imaging systems by using electrical colormasks.

Another object of the invention is to effect color correction inelectrophoretic imaging systems by altering the electric fieldintensities effecting the movement of the particles that form an image.

Another object of the invention is to alter the density of aphotoelectrophoretic image by altering the electric field effectingmigration of the pigments comprising the image.

v Yet another object of this invention is the improvement of thephotoelectrophoretic imaging process.

In the present invention a color image is composed from three separateyellow, magenta and cyan photoelectrophoretic ink images or colorseparation images that are transferred in registration to a recordmember. The three separation images are formed on injecting electrodesfrom yellow, magenta and cyan inks that are exposed to the blue, greenand red light, respectively, emitted from an original. The yellow andmagenta separation images are color corrected by increasing the electricfield applied across the ink in the areas that the pigment density is tobe reduced. The densityis reduced because the increased field increasesthe number of pigments migrating from the injecting electrode for agiven exposure. The electric field is increased by an electrostaticimage on a blocking electrode shaped in the configuration of a positiveimage of the green and/or redlight emitted by the original. Theseelectrostatic images are called electrical color masks.

DESCRIPTION OF THE DRAWINGS Other objects and features of the presentinvention will be apparent from a further reading of the presentdescription and from the drawings which are:

FIG. 1 is a schematic of a photoelectrophoretic imag ing systemutilizing an electrical color correcting mask.

FIG. 2 is a plot of the absorption characteristics of yellow, magentaand cyan pigments over the visible light spectrum.

DESCRIPTION OF THE INVENTION Curves 1, 2 and 3 in FIG. 2 depictgenerally the response of yellow, magenta and cyan photoelectrophoreticinks, respectively, to the visible light spectrum. These curves arehelpful in understanding the color subtraction mechanism used in thepresent photoelectrophoretic color imaging system. Ink pigments areattracted to an injecting electrode by the electric field and theymigrate toward the blocking electrode, i.e. are subtracted from theinjecting electrode, when exposed to light if they exchange charge withthe injecting electrode. Charge exchange occurs with those pigments inthe electric field that absorb the incident electromagnetic radiation.Consequently, yellow pigment is subtracted from the injecting electrodein the blueemitting areas of an original while magenta is subtractedfrom the green emitting areas and cyan from the red emitting areas.

The intensity or brightness of blue light, for example, is inverselyproportional to the quantity of yellow color in the original. The yellowremaining on the injecting electrode therefore is directly proportionalto the amount of yellow in the original. Actually, the quantity ofyellow pigment or-ink remaining on the injecting electrode is in errorbecause not all the blue light is absorbed by the yellow pigment, notall the pigments exchange charge with the injecting electrode, or ifthey do, they remain at the injecting'electrode for diverse reasons, andthe yellow particles absorb some radiation in the green and red portionsof the spectrum. The total error or deviation from the correctproportion between yellow pigment on the injecting electrode and yellowcolor in the original is herein referred to as the inherent error of theyellow pigment.

Similarly, green light impinging upon a magenta ink subtracts magentapigments from an injecting electrode by an'amount, at least ideally,that leaves a quantity directly proportional to the amount of magentacolor in the original. Again, the quantity of magenta pigment on theinjecting electrode differs from the idealbecau'se of the inherent errorof the magenta pigment, which is analogous to the inherent error of theyellow pigment. Likewise, the red light incident on a cyan ink subtractscyan pigments from the injecting electrode by an areas of a reproductioncontaining two or more pigments, a substantial portion of the colorerror is due to the fact that at least one pigment is responsive towavelengths outside its ideal spectral response region. By way ofexample, a color in an original that is a combination of 50 percentyellow and 50 percent magenta emits light proportional to one unit greenlight, one unit blue light and two units red light. The unit of bluelight drives (ideally) 50 percent of the yellow pigment from aninjecting electrode leaving a layer of yellow pigment behind directlyproportional to the amount of yellow in the original. The unit of greenlight drives (ideally) 50 percent of the magenta pigment from aninjecting electrode and the two units of red light drive (ideally) allthe cyan pigment from an injecting electrode. When the blue and greenseparation images are laid in registration the composite image is 50percent yellow and 50 percent magenta. Unfortunately, when thiscomposite image is viewed under a white light, it reflects (or transmitsif it is a transparency) light generally pro portional to 0.75 unitsblue light, 0.9 units green light and 1.9 units of red light. The lowoutput of blue light is due primarily to the fact that the magentapigment absorbs an appreciable amount of blue light and the resultantimage appears too yellow. This error is corrected by reducing the amountof yellow pigment wherever magenta pigment is also required. In thepresent example, removing 25 percent of the yellow pigment in the areasalso containing magenta pigment enables the reproduction to moreaccurately simulate the color of the original. Similar examples can beconstructed illustrating that yellow and cyan pigments and magenta andcyan pigments more accurately represent a like color combinations in anoriginal by reducing the density of the yellow and magenta pigments inthe areas also containing cyan pigment. With the aid of such examples,one skilled in the art can readily devise methods for calibrating theamount of one pigment that should be removed from areas containing apigment having a spectral response region that overlaps into that of thefirst pigment. Experience has shown that substantial amount of the colorerror in a reproduction is corrected by reducing the yellow pigmentdensity where there is magenta and/or cyan pigments and reducing magentapigment density where there is cyan pigment. An initial inspection ofthe overlap between the curves in FIG. 2 explains why the specificallymentioned corrections are so effective and further comparisons of thecurves suggests other density changing schemes that result in even morecolor correction and/or enhancement. 7

Turning now to FIG. 1, the system injecting electrode 5 includes thetransparent glass plate 6 overcoated with the transparent layer ofconducting material 7, e .g. tin oxide. The layer 8 represents amonochromatic photoelectrophoretic ink deposited by an inking roller,brush or other appropriate means on the injecting electrode. Theblocking electrode 9 includes a conductive cylindrical core 10 having anouter layer of electrically insulating material 11. The insulating layeris preferably a photoconductive material for reasons that will be givenshortly. The cylindrical blocking electrode, i.e. roller electrode, ispositioned to form a nip 12 with the injecting electrode. The nip is thearea defining an interface between the two electrodes whether or notthey are separated by an ink. The ink in the vicinity ofthe nip issubjected to an electric field'by reason of the ground potential 13coupled to the tin oxide layer of the injecting electrode and the groundreferenced power supply 14 coupled to the conductive core of theblocking electrode. The power supply 15 energizes the coratron 16 whichis used in forming the electrostatic color mask of the present inventionwhich is discussed more fully elsewhere.

The ink 8 on the injecting electrode is exposed to activatingelectromagnetic radiation by the exposure mechanism 17 that includes thelamp 19 and the lens 20. The transparency original 21 is a positivecolor image, i.e. transmits blue, green and red light emitted by thewhite light generator (lamp' 19) in inverse proportion to the density ofyellow, magenta and cyan pigment or dye in it. For example, a yellow Aon a cyan background ideally transmits no blue light in the area of theA and all the blue light in the background area. Lens 20 projects thelight transmitted by the original to the layer of ink on the injectingelectrode.

The electric field established between the two electrodes is of apolarity causing the ink particles to adhere to the injecting electrode.If the pigments in the nip are exposed to light within their absorptionspectrum, they experience a change in polarity and migrate toward theblocking electrode. Consequently, a two dimensional positive ink imageis formed on the injecting electrode in a line by'line fashion as theroller electrode rotates and travels over the injecting electrode. Anegative ink image is also formed on. the blocking electrode and it maybe used with other ink images toform a composite image as wouldbeunderstood by the present disclosure and theteachings of the colorcomposing art. For simplicity, the present disclosure is limited tocolor composition with separation images formed on the injectingelectrode.

An image formed on an injecting electrode in the above fashion may betransferredto a record member:

by several well known techniques Forexample, a roller electrode suchasroller 91iscoupled to a voltage polari- Any transfer apparatus employedis arranged to receive a plurality of images in the exact same location,i.e. in registration. The record membernormally includes a wood or clothfiber paper.

With thisbackground, thethree step method of uncorrected colorcomposition is easily. understood- A yellow pigment ink is deposited onan injecting electrode 9 and exposed to radiation in imagewiseconfigwration by exposure mechanism 17. As-the roller moves over thetransparent electrode (here .theinjecting electrode), the blue-lighttransmittedrby original. 21 activates the yellow pigments in thevicinity of nip 12 causing them to migrate away leaving yellow pigmentin quantities directly proportional to theyellow color in the original21. This blue separationimage, i.e. the yellow pigment image on theinjecting electrode, is transferred to a record member and the surfacesof the injecting and blocking electrodes are cleaned. Magenta ink isdeposited on the injecting electrode and the roller electrode ispassedover it. The green light transmitted by original 21 activates thepigments in the vicinity of the nip causing them to migrate to theblocking electrode. This green separation image is transferred to therecord member in registration with the blue separation image transferredearlier. Again, the injecting and blocking electrodes are cleaned and acyan ink is deposited onto the injecting electrode. As the rollerelectrode travels over the injecting electrode, the red light activatesthe cyan pigments in the vicinity of the nip causing them to migratetoward the blocking electrode. The red separation image on the injectingelectrode is transferred in registration to the record member bearingthe blue and green separation images thereby completing the formation ofa full color reproduction on the record member. A fourth separationimage made with black pigment ink exposed to light transmitted through aneutral density filter could also be formed in a similar fashion andtransferred in registration with the other separation images. Thisfourth image primarily acts to increase the contrast between colors. Ofcourse, other known color composing techniques or steps known in the artmay be utilized if sodesired. l

The density of the above described separation images may be altered toeffect color correction or to enhance one color relative tothe others byaltering the electric field to which the inks are subjected. Thealteration is performed by creating an electrostatic.

charge patternon the surface of the blocking electrode, i.e. a surfacecapable of sustaining the electrostatic charge. The electrostatic chargemay either increase or decrease the field depending upon whethermigration from thetransparent electrode is to be increased or decreasedfor a particular color correcting or enha nc--- ing scheme. For thepresent color correction scheme,-

the electrostatic charge increases the field in selected areas toincrease themigration of pigment from theinjecting electrode. Theselected areas are the areas in the original containing magenta and/orcyan for the blue-separation image and the areas in the originalcontaining cyan for the green separation image.

lnthe embodiment shown in FIG. 1, the insulating layer llon the rollerelectrode is a photoconductive material such as amorphous selenium. Byway of example, thetin oxide layer 7 on the injecting electrode'iscoupled to ground potential'and the core 10 of the roller 9 is coupledto a high negative potential B. The B voltage gradient establishes anelectric field between thetwo electrodes of sufficient magnitude toenable the photoelectrophoretic process to work. The photoconductivesurface-ll is charged in the dark by the corotron 16 which is coupled toa voltage potential that is the' sum of B and A. The A voltage gradientbetween the photoconductive layer 11 and corotron 16 results insubstantially a uniform deposition of electrons on layer 11. The voltagepotential of the deposited charge is related to the density of theelectrons on layer 11. The'electron density is dependent upon the rateof electron flow to the surface 11 and the rate of rotation of theroller 9 both of which are controlled by well known techniques. At anyrate, the corotron charges the photoconductive surface to some potentialnear -A thereby increasing the voltage gradient, i.e. the electricfield, between the two electrodes and 9.

Next, the charged photoconductive surface 11 is exposed to a lightpattern of the green and/or red light emitted from the original 21depending upon whether the color mask is for a yellow or magenta ink.(The spectral response of selenium photo-conductors, for example,extends from the blue through the red regions of the visible lightspectrum when the selenium includes additives such as tellurium, arsenicand/or sulfur.) The intensities of the green and red light are inverselyproportional to the amount of magenta or cyan in the original,respectively. The light renders the photoconductor conductive therebybleeding the charge from the surface of the photoconductor in areascorresponding to areas in the original that contain little of either themagenta and/or cyan colors. In other words, the electrostatic charge orimage on the photoconductor after exposure represents a positive imageof the magenta and/or cyan colors in the original when the color mask isfor a yellow ink or a positive image of the cyan color in the originalif the color mask is for a magenta ink. In each case, ink on theinjecting and blocking electrodes is a shield that prevents the lightemitted by exposure mechanism 17 from erasing the electrostatic image onlayer 11.

The relative polarities of the voltages in the above example areillustrative only. Other voltage polarities can be used to effect thedesired pigment migrations and the electrostatic charge deposited onlayer 11 may include positive ions generated by a corotron rather thanelectrons.

Once the electrostatic color mask is formed on the trode is aligned inregistration with the image projected to the ink.by exposure mechanism17. When the roller is moved across the injecting electrode, the inkpigment migration is increased in the areas of the electrostatic imagebecause the field strength is greater in those areas. The increasedmigration reduces the density of thepigment on the injecting electrodein the desired areas in the manner that has been repeatedly explained.

The mask exposuremechanism 22 is used to expose photoconductive layer 11to the desired light patterns I and it includes the lamp 23, aperturelens stop 24, lens 25 and filter 26. Lamp 23 floods the positivetransparency original 28 (the same original as transparency 21 but movedto the location of original 28) with white light. The light transmittedby original 28 passes through the slit 29 in the lens stop and isfocused onto the periphery of the roller 9 by the lens 25. Anappropriate filter 26 is inserted next to the lens to pass green and redlight for masking a yellow ink or to pass red light for masking amagenta ink. The lamp, lens stop, lens filter and the roller electrodeare all part of a group 30 that are (neglecting the rotation of theroller) stationaryrelative to one another. Consequently, moving theoriginal 28 relative to the lens stop 24 and at a speed synchronizedwith the rotational velocity of the roller electrode creates theelectrostatic image 31 by discharging the uniform electrostatic chargein the areas struck by light.

The formation of the electrostatic image has been described as occuringseparately from the exposure of I surface of the blocking electrode, theblocking elec- LII an ink by the exposure mechanism 17. The twooperations can occur simultaneously. For simplicity, the simultaneousoperation is illustrated for the case where two identical originals 21and 28 are available. if only one original is available relativelycomplex exposure apparatus is used to project a light image of thesingle original to both the ink and the photoconductor. in either case,the electrostatic mask must be substantially in registration with theimage projected onto the ink. The electrostatic image 31 is formed onthe roller 9 at a point in its travel such that line 32 in image 31 willcoincide with a line 33 in the projected light image 34 when the rollerhas rotated and translated through a distance to put both lines 32 and33 in the nip 12. The lines 32 and 33, of course, represent the sameline 36 in both the originals 21 and 28. In the system of FIG. 1, thisis accomplished by moving the apparatus in group 30 past astationaryoriginal 28, a stationary injecting electrode 5, a stationary exposuremechanism 17 and a stationary original 21. The originals 28 and 21 areoriented to compensate for image inversion and reversion due to thelenses 20 and 25 and the fact that the same image is impressed on twosides of the ink layer 8; namely, from'above the ink on thephotoconductive layer 11 and from below the ink through the transparentelectrode.

The electrostatic image 31 can be formed on surfaces other than aphotoconductive surface. For example, layer 11 may comprise anon-photoconductive electrically insulating material. in this case, agrounded conductive stencil is placed over layer 11 and the roller isrotated under the corotron 16. The stencil prevents charge accumulationon layer 11 in the non-image areas of the stencil and allows chargeaccumulation in the image areas, i.e. the cutout areas of the stencil.Alternatively, a charge pattern can be formed on a photoconductor and betransferred to a non-photoconductive layer 11 by charge inductiontechniques. Also, the electrostatic image may be created directly on theink'layer 8 or on an insulating layer between the ink and the injectingelectrode.

Other variations to the disclosed embodiments will occur to thoseskilled in the art. For example, both the injecting and blockingelectrodes may be flat plates, both may be rollers or the injectingelectrode may be a roller and the blocking electrode a plate. Also, theblocking electrode may be transparent rather than the injectingelectrode. Quite obviously, exposure mechanisms may be used forprojecting images of opaque originals. Many other alterations can bemade and all are intended to be encompassed within the scope ofthepresent invention as long as the density of an image on one of theelectrodes is changed by altering the electric field effecting the inkparticle migration.

What is claimed is:

1. An imaging method comprising a. providing a monochromaticphotoelectrophoretic ink including electrically photosensitive particlessuspended in an electrically insulating fluid, said ink being primarilysensitive to radiation within a first region of the electromagneticspectrum;

b. providing a pair of electrodes at least one of which is at leastpartially transparent to radiation within said first region and one ofsaid electrodes having an insulating surface;

c. arranging a layer of said monochromatic ink between said electrodes;

(1. establishing an electric field across said electrodes;

e. exposing said monochromatic ink to a pattern of activating radiationof wavelengths within said first region emitted by an original, whereinsaid original is capable of emitting wavelengths of radiation bothwithin said first region and outside of said first region; and

f. locating a pattern of electrostatic charge on said insulating surfaceof said electrode, said charge pattern being in a configuration of thewavelengths outside of said first region emitted by an original, whereinsaid charge pattern on said insulating surface and said pattern ofactivating radiation to which said ink is exposed are positioned inregistration with each other whereby the density of the ink particlescomprising images on the electrodes is altered in the areas of theelectrostatic charge.

2. The method of claim 1 wherein said insulating layer comprises aphotoconductive material and said electrostatic charge pattern is formedby uniformly charging said photoconductive insulating layer and exposingthe charged layer to electromagnetic radiation within said first regionemitted by said original.

3. The method of claim 1 further including transferring an image formedon one of said electrodes to a transfer member.

4. The method of claim 3 further including repeating the steps of claim3 at least one additional time, each subsequently formed image being ofa different color than said first image and each subsequently formedimage being comprised of a monochromatic ink primarily sensitive toradiation within a region of said spectrum different from said firstregion, and wherein for each additional image:

the exposure in step (e) is carried out with activating radiation ofwavelengths emitted by said original within the region to which said inkis primarily sensitive,

the electrostatic charge pattern recited in step (f) is in aconfiguration of the wavelengths emitted by said original outside ofsaid region to which said ink is primarily sensitive, and

each subsequently transferred image is transferred in registration withsaid first transferred image.

5. In a three step photoelectrophoretic color imaging method whereinyellow, magenta and cyan monochromatic pho toelectrophoretic inks areseparately exposed in an electric field established between twoelectrodes one of which has an insulating surface to blue, green and redradiation, respectively, emitted from an original to form blue, greenand red separation images that are transferred in registration to arecord member, each said monochromatic ink including electricallyphotosensitive particles suspended in an electrically insulating fluid,the improvement comprising locating an electrostatic charge pattern onthe insulating surface of said electrode during the formation of atleast one separation image to alter the density of the ink in aseparation image, said electrostatic charge pattern being in aconfiguration of wavelengths emitted by said original outside of thewavelengths emitted by said original within the region to which said inkis primarily sensitive.

6. The method of claim 5 wherein electrostatic charge in theconfiguration of the green and red light emitted from said original islocated adjacent the yellow ink during formation of the blue separationimage and electrostatic charge in the configuration of the red lightemitted from said original is adjacent the magenta ink during formationof the green separation image.

7. The method of claim 5 wherein said insulating surface isphotoconductive and further including charging said photoconductivesurface and exposing said charged surface to electromagnetic radiationto dissipate charge from the photoconductive surface in

1. An imaging method comprising a. providing a monochromaticphotoelectrophoretic ink including electrically photosensitive particlessuspended in an electrically insulating fluid, said ink being primarilysensitive to radiation within a first region of the electromagneticspectrum; b. providing a pair of electrodes at least one of which is atleast partially transparent to radiation within said first region andone of said electrodes having an insulating surface; c. arranging alayer of said moNochromatic ink between said electrodes; d. establishingan electric field across said electrodes; e. exposing said monochromaticink to a pattern of activating radiation of wavelengths within saidfirst region emitted by an original, wherein said original is capable ofemitting wavelengths of radiation both within said first region andoutside of said first region; and f. locating a pattern of electrostaticcharge on said insulating surface of said electrode, said charge patternbeing in a configuration of the wavelengths outside of said first regionemitted by an original, wherein said charge pattern on said insulatingsurface and said pattern of activating radiation to which said ink isexposed are positioned in registration with each other whereby thedensity of the ink particles comprising images on the electrodes isaltered in the areas of the electrostatic charge.
 2. The method of claim1 wherein said insulating layer comprises a photoconductive material andsaid electrostatic charge pattern is formed by uniformly charging saidphotoconductive insulating layer and exposing the charged layer toelectromagnetic radiation within said first region emitted by saidoriginal.
 3. The method of claim 1 further including transferring animage formed on one of said electrodes to a transfer member.
 4. Themethod of claim 3 further including repeating the steps of claim 3 atleast one additional time, each subsequently formed image being of adifferent color than said first image and each subsequently formed imagebeing comprised of a monochromatic ink primarily sensitive to radiationwithin a region of said spectrum different from said first region, andwherein for each additional image: the exposure in step (e) is carriedout with activating radiation of wavelengths emitted by said originalwithin the region to which said ink is primarily sensitive, theelectrostatic charge pattern recited in step (f) is in a configurationof the wavelengths emitted by said original outside of said region towhich said ink is primarily sensitive, and each subsequently transferredimage is transferred in registration with said first transferred image.5. In a three step photoelectrophoretic color imaging method whereinyellow, magenta and cyan monochromatic photoelectrophoretic inks areseparately exposed in an electric field established between twoelectrodes one of which has an insulating surface to blue, green and redradiation, respectively, emitted from an original to form blue, greenand red separation images that are transferred in registration to arecord member, each said monochromatic ink including electricallyphotosensitive particles suspended in an electrically insulating fluid,the improvement comprising locating an electrostatic charge pattern onthe insulating surface of said electrode during the formation of atleast one separation image to alter the density of the ink in aseparation image, said electrostatic charge pattern being in aconfiguration of wavelengths emitted by said original outside of thewavelengths emitted by said original within the region to which said inkis primarily sensitive.
 6. The method of claim 5 wherein electrostaticcharge in the configuration of the green and red light emitted from saidoriginal is located adjacent the yellow ink during formation of the blueseparation image and electrostatic charge in the configuration of thered light emitted from said original is adjacent the magenta ink duringformation of the green separation image.